The present invention relates to a drive navigation apparatus for a vehicle, and, more particularly, to an improvement in display method of a navigation apparatus which is provided with a display device.
When a vehicle is driven to a destination, the driver of the vehicle operates the accelerator pedal and the like in the interior of the vehicle which is isolated from the outside, depending upon only a map, signposts and scenery. That is, although an excessive quantity of information is nowadays supplied due to the advancement of computer and data communication technology, the lacking of information necessary for the driver to navigate the vehicle has not been improved from the time when the automobile was invented.
Accordingly, a driving support system so-called a navigation system has, at long last, been developed recently.
The navigation system is classified into a system (to be called an "earth magnetism system" hereinafter) in which the horizontal component of the earth magnetism vector is detected so as to be used as the azimuth parameter and a system (to be called an "inertia navigation system" hereinafter) which employs a gas rate gyro which utilizes the inertia of helium gas as an azimuth sensor so as to detect the azimuth. Furthermore, a novel navigation system, that is, a GPS (Global Positioning System) has attracted attention recently in which three or more orbit satellites (also called "navigation satellites") are used so that an accurate position on the earth can be determined. The above-described systems are classified depending upon the methods of detecting and enabling a user to recognize the position of the vehicle while driving the vehicle. Therefore, the above-described systems basically have common portions.
The navigation system comprises a CD-ROM reproducing device which stores a plurality of color map data items as drive navigation information for a vehicle. Furthermore, the navigation system comprises means for inputting the destination, means for locating the destination thus set in the map data, and a sensor of the above-described earth magnetism type as the means for recognizing the vehicle position. As a result, the most suitable route to the destination between the current position of the vehicle and the destination is displayed on a CRT display (see, for example, Japanese Patent Laid-Open No. 61-20921).
Thanks to the vehicle navigation apparatus thus constituted, the driver can easily reach the destination by operating the vehicle while tracing the most suitable route (road) shown on the map displayed on the CRT display.
A mark showing the position of the user's vehicle is displayed on the route shown in the frame of the display, the mark being shown in a plot manner and arranged to be moved in synchronization with the movement of the vehicle.
The navigation system is provided with a display device for displaying the mark of the current position of the vehicle together with map image. The mark and the map image have been conventionally displayed in the following methods:
I. The background map image is moved in accordance with the movement of the vehicle while fixing the vehicle position mark at a predetermined position in the display frame.
II. The vehicle position mark is moved in accordance with the movement of the vehicle while fixing the background map image.
According to either of the above-described methods, the positional relationship between the vehicle position and the map image is changed in accordance with the movement of the vehicle.
Since a driver tends to feel that the outside is changed with respect to the vehicle as an alternative to the feeling that the vehicle is moved with respect to the outside, an evaluation has been made that the above-described method I in which the background map image is changed can be easily used by a driver.
However, in the above-described system I in which the background image is moved in accordance with the movement of the vehicle, the movement of the map becomes too fast when the vehicle speed has been raised, causing a problem to arise in that the driver cannot easily recognize the position of the vehicle. In particular, the map image is displayed in the smaller reduction rate, the more the visibility deteriorates since the map movement speed becomes higher in inverse proportion to the reduction ratio of the map. That is, the scroll speed of the display has a certain limit to allow for the driver to recognize the map image. Therefore, it cannot be raised over a certain speed. If the scroll speed is raised neglecting the visibility, the scroll speed encounters a physical limit due to the limit of the processing speed of the image processing computer.
As described above, the scroll speed has limits so as to make the display easily recognizable and due to the image processing speed of the computer.
As a result, if the vehicle speed is too high, the scroll speed which corresponds to the vehicle speed sometimes exceeds the above-described speed limits. It leads to a fact that vehicle position runs out of the display frame, causing a problem to arise in that the navigation apparatus cannot be used.